Solar heat-reflective roofing granules, solar heat-reflective shingles and process for producing same

ABSTRACT

Solar-reflective roofing granules having improved solar heat-resistance are formed by coating colored mineral particles with a coating composition including titanium dioxide nanoparticles.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of pending U.S. patent applicationSer. No. 12/057,131, filed Mar. 27, 2008, which claimed the priority ofU.S. Patent Application No. 60/909,892 filed Apr. 3, 2007 and U.S.Patent Application Ser. No. 60/909,616 filed Apr. 2, 2007.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to asphalt roofing shingles, andprotective granules for such shingles, and processes for making suchgranules and shingles.

2. Brief Description of the Prior Art

Pigment-coated mineral rocks are commonly used as color granules inroofing applications to provide aesthetic as well as protectivefunctions to the asphalt shingles. Roofing granules are generally usedin asphalt shingle or in roofing membranes to protect asphalt fromharmful ultraviolet radiation.

Roofing granules typically comprise crushed and screened mineralmaterials, which are subsequently coated with a binder containing one ormore coloring pigments, such as suitable metal oxides. The binder can bea soluble alkaline silicate that is subsequently insolubilized by heator by chemical reaction, such as by reaction between an acidic materialand the alkaline silicate, resulting in an insoluble colored coating onthe mineral particles. Preparation of colored, coated roofing granulesis disclosed for example, in U.S. Pat. No. 2,981,636 of Lodge et al. Thegranules are then employed to provide a protective layer on asphalticroofing materials such as shingles, and to add aesthetic values to aroof.

Pigments for roofing granules have usually been selected to provideshingles having an attractive appearance, with little thought to thethermal stresses encountered on shingled roofs. However, depending onlocation and climate, shingled roofs can experience very challengingenvironmental conditions, which tend to reduce the effective servicelife of such roofs. One significant environmental stress is the elevatedtemperature experienced by roofing shingles under sunny, summerconditions. Although such roofs can be coated with solar reflectivepaint or coating material, such as a composition containing asignificant amount of titanium dioxide pigment, in order to reduce suchthermal stresses, this utilitarian approach will often prove to beaesthetically undesirable, especially for residential roofs.

Asphalt shingles coated with conventional roofing granules are known tohave low solar heat reflectance, and hence will absorb solar heatespecially through the near infrared range (700 nm-2500 nm) of the solarspectrum. This phenomenon is increased as the granules covering thesurface become dark in color. For example, while white-colored asphaltshingles can have solar reflectance in the range of 25-35%, dark-coloredasphalt shingles can only have solar reflectance of 5-15%. Furthermore,except in the white or very light colors, there is typically only a verysmall amount of pigment in the conventional granule's color coating thatreflects solar radiation well. As a result, it is common to measuretemperatures as high as 77° C. on the surface of black roofing shingleson a sunny day with 21° C. ambient temperature. Absorption of solar heatmay result in elevated temperatures at the shingle's surroundings, whichcan contribute to the so-called heat-island effects and increase thecooling load to its surroundings.

One approach to addressing this problem is suggested in U.S. PatentApplication Publication No. 2003/0068469 A1 and U.S. Pat. No. 7,238,408which disclose an asphalt-based roofing material comprising matsaturated with asphalt coating and a top coating having a top surfacelayer that has a solar reflectance of at least 70%. Another approach issuggested in U.S. Patent Application Publication 2003/0152747 A1 andU.S. Pat. No. 6,933,007 which disclose the use of novel granules withsolar reflectance greater than 55% and hardness greater than 4 in Moh'sscale to enhance the solar reflectivity of asphalt based roofingproducts.

There is a continuing need for roofing materials, and especially asphaltshingles, that have improved resistance to thermal stresses whileproviding an attractive appearance. In particular, there is a need forroofing granules that provide increased solar heat reflectance to reducethe solar absorption of the shingle.

SUMMARY OF THE INVENTION

The present invention provides roofing granules that provide increasedsolar heat reflectance, as well as a process for preparing such roofinggranules, and asphalt shingle roofing products incorporating suchroofing granules.

The present invention provides, in several aspects, solarheat-reflective roofing granules comprising a base particle comprisingan inert mineral, with a first coating on the base particle, and asecond coating on the first coating, wherein at least one of the firstcoating and the second coating comprises a solar heat-reflectivecoating. In another aspect, the present invention provides solarheat-reflective roofing granules comprising a solar heat-reflective baseparticle, and at least one color coating.

In a first aspect, the present invention provides solar heat-reflectiveroofing granules comprising a base particle comprising an inert mineral,a first or inner coating on the base particle, and a second or outercoating on the first coating. In this aspect, the second coating isselected from the group consisting of coatings comprising solarheat-reflective nano-pigment particles and coatings comprisingmultilayer infrared-reflective films. Preferably, the second coating hasan average incident radiation transmission coefficient of at least 80percent in the range 400 nanometers to 800 nanometers. Thus, the secondcoating is preferably substantially transparent or translucent in thevisible range.

In one presently preferred embodiment of this first aspect of thepresent invention, the first coating on the base particle comprises ametal oxide colorant. Thus, in this aspect, the metal oxide colorantprovides color to the first or inner coating, which can be seen throughthe substantially transparent second or outer coating.

Preferably, in this first aspect of the present invention, the secondcoating comprises solar heat-reflective nanoparticles that are highlyreflective in the near infrared. It is preferred that the second coatingcomprises titanium dioxide nanoparticles having an average particle orcrystal size of less than about 100 nanometers, and more preferably,titanium dioxide nanoparticles having an average particle size of lessthan about 50 nanometers. In the alternative, in this first aspect ofthe present invention, it is preferred that the second coating comprisea multilayer infrared-reflective film, at least one layer of themultilayer film being formed from a metal selected from the groupconsisting of silver, gold and copper. In this alternative, it ispreferred that the thickness of the second coating be less than about 50nanometers.

Preferably, in this first aspect of the solar-reflective roofinggranules of the present invention, the granules further comprise ahydrophobic coating on the second or outer coating.

In a second aspect, the present invention provides solar heat-reflectiveroofing granules including a base particle, which itself comprises aninert mineral, a first or inner coating on the base particle, and asecond or outer coating on the first coating. In this aspect the firstcoating comprises a first coating binder and at least one solarreflecting pigment particulate having an average reflectance of greaterthan about 60 percent in the wavelength range of from about 700 to 2500nanometers, and the second coating comprises a second coating binder andat least one solar heat-transparent nano-pigment having an averageparticle size of less than about 200 nanometers and an absorbency ofless than about 20 percent in the wavelength range of from 700 to 2500nanometers. Preferably, the solar reflecting pigment particles have anaverage solar reflectivity of at least 80 percent in the wavelengthrange from 700 to 2500 nanometers. Preferably, the solar reflectingpigment particles are selected from the group consisting of titaniumdioxide, zinc dioxide, and zinc sulfide.

Preferably, in this second aspect the second coating binder comprises ametal silicate binder having a refractive index of less than about 1.50.Preferably, the second coating binder comprises a silicate coatingbinder including at least one low atomic weight element, other thanoxygen or hydrogen, having an average atomic weight less than theaverage atomic weight of silicon. It is preferred that the at least onelow atomic weight element be present in sufficient amount in the coatingbinder to reduce the refractive index by at least about 0.003 unitscompared with a coating binder without the at least one low atomicweight element but otherwise having the same proportional elementalcomposition.

Optionally, the second coating can further comprise at least onesupplementary pigment having a particle size of greater than about 200nanometers and an average absorbency of less than about 20 percent inthe wavelength range of from 700 to 2500 nanometers. Preferably, the atleast one supplementary pigment is selected from the group consisting ofpearlescent pigments, light-interference platelet pigments, ultramarineblue, ultramarine purple, cobalt chromite blue, cobalt aluminum blue,chrome titanate, nickel titanate, cadmium sulfide yellow, cadmiumsulfoselenide orange, phthalo blue, phthalo green, quinacridone red,diarylide yellow, and dioxazine purple.

It is further preferred in this second aspect that the at least onenano-pigment have an average particle size of from about 20 to 150nanometers. Preferably, the nano-pigment is selected from the groupconsisting of iron oxides, metal titanates, chromium oxides, zincferrites, mixed metal oxides, titanium dioxide, zinc oxides, copperoxides, vanadium oxide, magnesium oxide and the halogen adducts.Optionally, the nano-pigment is selected from the group of pigments thathave strong near infrared absorbency in macro-pigment form. In thiscase, it is preferred that the at least one nano-pigment is selectedfrom the group consisting of carbon black, bone black, copper chromiteblack, iron oxide black, and KFe₂(CN)₆.H₂O (“iron blue”).

In a third aspect, the present invention provides solar heat-reflectiveroofing granules comprising a solar-reflective inert base particle, anda color coating over the solar-reflective base particle. The colorcoating includes a binder and at least one nano-pigment having aparticle size of less than about 200 nanometers and an averageabsorbency of less than about 20 percent in the wavelength range of from700 to 2500 nanometers. Preferably, the absorbency of the nano-pigmentis reduced by at least 50 percent of the absorbency of the correspondingmacro-pigment in the same range. Preferably, the solar-reflective inertbase particles have an average solar reflectivity of at least 60%.Preferably, the solar-reflective inert base particles are selected fromthe group consisting of slate, feldspathic rock, plagioclase rock, chertrock, aluminum oxide, mullite, ceramic grog, crushed porcelain,white-pigmented glass, copper, and zinc.

Preferably, the color coating comprises a metal silicate binder having arefractive index of less than about 1.50. It is also preferred that themetal silicate binder comprises a silicate coating binder including atleast one low atomic weight element, other than oxygen or hydrogen,having an average atomic weight less than the average atomic weight ofsilicon. Preferably, the at least one low atomic weight element ispresent in sufficient amount in the coating binder to reduce therefractive index by at least about 0.003 units compared with a coatingbinder without the at least one low atomic weight element but otherwisehaving the same proportional elemental composition.

Preferably, the color coating further comprising at least onesupplementary pigment having a particle size of greater than about 200nanometers and an average absorbency of less than about 20 percent inthe wavelength range of from 700 to 2500 nanometers. Preferably, the atleast one supplementary pigment is selected from the group consisting ofpearlescent pigments, light-interference platelet pigments, ultramarineblue, ultramarine purple, cobalt chromite blue, cobalt aluminum blue,chrome titanate, nickel titanate, cadmium sulfide yellow, cadmiumsulfoselenide orange, phthalo blue, phthalo green, quinacridone red,diarylide yellow, and dioxazine purple.

Preferably, in this third aspect the at least one nano-pigment has anaverage particle size of from about 20 to 150 nanometers. It is furtherpreferred that the nano-pigment be selected from the group consisting ofiron oxides, metal titanates, chromium oxides, zinc ferrites, mixedmetal oxides, titanium dioxide, zinc oxides, copper oxides, vanadiumoxide, magnesium oxide and the halogen adducts.

Optionally, the nano-pigment can be selected from the group of pigmentsthat have strong near infrared absorbency in macro-pigment form.Preferably, the at least one nano-pigment is selected from the groupconsisting of carbon black, bone black, copper chromite black, ironoxide black (“magnetite”), and KFe₂(CN)₆.H₂O (“iron blue”).

In a fourth aspect, the present invention provides solar heat-reflectiveroofing granules comprising an inert mineral base particle; and a solarheat-reflective, color coating over the base particle, the solarheat-reflective, color coating comprising a binder and at least onenano-pigment having a particle size of less than about 200 nanometersand a reduced absorbency in the wavelength range of from 700 to 2500nanometers, the absorbency being reduced by at least 50 percent of theabsorbency of the corresponding macro-pigment in the same range, and atleast one solar heat-reflective pigment. Preferably, the at least onenano-pigment has an average particle size of from about 20 to 150nanometers. Optionally, the nano-pigment is selected from the group ofpigments that have strong near infrared absorbency in macro-pigmentform. In this case, it is preferred that the at least one nano-pigmentbe selected from the group consisting of carbon black, bone black,copper chromite black, iron oxide black, and KFe₂(CN)₆.H₂O (“ironblue”).

The present invention also provides processes for preparing solarheat-reflective roofing granules. In one aspect, the process of thepresent invention comprises providing base particles comprising an inertmineral, coating the base particles with a first coating composition,curing the first coating composition to form intermediate particles,coating the intermediate particles with a second coating composition,and curing the second coating composition. In one embodiment of theprocess, the first coating composition includes a colorant, such asconventional metal oxide colorants and/or nano-pigment colorants, andthe second coating composition is selected from the group consisting ofcoating compositions comprising nanoparticles of at least one nearinfrared-reflective substance, and coating compositions comprisingmultilayer infrared-reflective films. In another embodiment, the firstcoating composition comprises nanoparticles of at least one nearinfrared-reflective substance such as nanoparticle titanium dioxide, andthe second coating composition comprises colored nano-pigment that issubstantially transparent to the near infrared spectrum.

In addition, the present invention provides a sheet roofing product,such as for example roofing shingles, including a bituminous base andsolar heat-reflective roofing granules according to the presentinvention. In one aspect the solar-reflective roofing granules comprisesa base particle comprising an inert mineral, a first coating on the baseparticle, and a second coating on the first coating. The second coatingis selected from the group consisting of coatings comprisingnanoparticles of at least one solar heat-reflective substance; andcoatings comprising multilayer infrared-reflective films. Preferably,the second layer has an incident radiation transmission coefficient ofat least 80 percent in the range 400 nanometers to 800 nanometers. Thefirst coating on the base particle preferably comprises a metal oxidecolorant. In one embodiment of the sheet roofing product according tothe present invention the second coating comprises titanium dioxidenanoparticles having an average particle or crystal size of less thanabout 100 nanometers, and more preferably, less than about 50nanometers. In another embodiment of the sheet roofing product accordingto the present invention, the second coating comprises a multilayerinfrared-reflective film, at least one layer of the multilayer filmbeing formed from a metal selected from the group consisting of silver,gold and copper. In this case, it is preferred that the thickness of thesecond coating be less than about 50 nanometers.

The present invention also provides a process for producinginfrared-reflective roofing shingles, as well as the shinglesthemselves. This process comprises producing infrared-reflective roofinggranules using the process of this invention, and adhering the granulesto a shingle stock material.

The colored, infrared-reflective roofing granules prepared according tothe process of the present invention can be employed in the manufactureof infrared-reflective roofing products, such as infrared-reflectiveasphalt shingles and roll goods, including bituminous membrane rollgoods. The colored, infrared-reflective granules of the presentinvention can be mixed with conventional roofing granules, and thegranule mixture can be embedded in the surface of bituminous roofingproducts using conventional methods. Alternatively, the colored,infrared-reflective granules of the present invention can be substitutedfor conventional roofing granules in manufacture of bituminous roofingproducts, such as asphalt roofing shingles, to provide those roofingproducts with solar-reflectance.

The present invention also provides processes for preparing solarheat-reflective roofing granules. In one aspect, the present processincludes the steps of (a) providing base particles comprising an inertmineral, (b) coating the base particles with a first or inner coatingcomposition; (c) curing the first coating composition to formintermediate particles; (d) coating the intermediate particles with asecond or outer coating composition, and (e) curing the second coatingcomposition. In this process, the second coating composition is selectedfrom the group consisting of (1) coating compositions comprisingtitanium dioxide nanoparticles; and (2) coating compositions comprisingmultilayer infrared-reflective films.

The present invention also provides a sheet roofing product, such as anasphalt shingle, including a bituminous base and the solarheat-reflective roofing granules provided by the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration of the structure of a section of asolar heat-reflective roofing granule according to a first embodiment ofthe present invention.

FIG. 2 is a schematic illustration of the structure of a section of asolar heat-reflective roofing granule according to a second embodimentof the present invention.

FIG. 3 is a schematic illustration of the structure of a section of asolar heat-reflective roofing granule according to a third embodiment ofthe present invention.

FIG. 4 is a schematic illustration of the structure of a section of asolar heat-reflective roofing granule according to a fourth embodimentof the present invention.

FIG. 5 is a schematic illustration of the structure of a section of asolar heat-reflective roofing granule according to a fifth embodiment ofthe present invention.

FIG. 6 is a schematic illustration of the structure of a section of asolar heat-reflective roofing granule according to a sixth embodiment ofthe present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Infrared-reflective or solar heat-reflective granules of the presentinvention can be prepared through traditional granule coloring methods,such as those disclosed in U.S. Pat. No. 2,981,636, incorporated hereinby reference.

Suitable inert base particles, for example, mineral particles with sizepassing #8 US mesh and retaining on #70 US mesh, can be coated with acombination the metal-silicate binders, kaolin clay, and reflectivepigments, or in combination of other color pigments to reach desirablecolors, followed by a heat treatment to obtain a durable coating.

Such a coating process can be repeated to form multiple coatings tofurther enhance the color and solar heat reflection.

As used in the present specification, “colored” means having an L* valueof less than 85, preferably less than 55, even more preferably less than45, when measured using a HunterLab Model Labscan XE spectrophotometerusing a 0 degree viewing angle, a 45 degree illumination angle, a 10degree standard observer, and a D-65 illuminant. “Colored” as so definedis intended to include relatively dark tones.

As used in the present specification and claims, “infrared-reflectivefunctional pigment” denotes a pigment selected from the group consistingof light-interference platelet pigments including mica,light-interference platelet pigments including titanium dioxide,mirrorized silica pigments based upon metal-doped silica, and alumina.As used in the present specification and claims, “granule coloringpigment” denotes a conventional metal oxide-type pigment employed tocolor roofing granules. UV-stabilized dyes are dye compositionsformulated with uv-stabilization materials. As used in the presentspecification and claims, the near infrared range (“NIR”) of solarspectrum means the spectral range from about 700 nm to about 2500 nm.

As used in the present specification and claims, the strength in colorspace E* is defined as E*=(L*²+a*²+b*²)^(1/2), where L*, a*, and b* arethe color measurements for a given sample using the 1976 CIE L*a*b*color space. The total color difference ΔE* is defined asΔE*=(ΔL*²+Δa*²+Δb*²)^(1/2) where ΔL*, Δa*, and Δb* are respectively thedifferences in L*, a* and b* for two different color measurements.

As used in the present specification and claims, “nanoparticle” means aparticle having an average particle size of less than about 200nanometers. As used in the present specification and claims,“nano-pigment” means a pigment particle having an average particles sizeless than about 200 nanometers.

The inert base particles or cores employed in the process of the presentinvention are preferably chemically inert materials, such as inertmineral particles. The mineral particles, which can be produced by aseries of quarrying, crushing, and screening operations, are generallyintermediate between sand and gravel in size (that is, between about #8U.S. mesh and #70 U.S. mesh), and more preferably with sizes rangingfrom #8 U.S. mesh to #40 U.S. mesh. Preferably, the mineral particleshave an average particle size of from about 0.2 mm to about 3 mm, andmore preferably from about 0.4 mm to about 2.4 mm.

In particular, suitably sized particles of naturally occurring materialssuch as talc, slag, granite, silica sand, greenstone, andesite,porphyry, marble, syenite, rhyolite, diabase, greystone, quartz, slate,trap rock, basalt, and marine shells can be used, as well as recycled ormanufactured materials such as propant bodies, crushed bricks, concrete,porcelain, fire clay, and the like. Other types of cores can also beused, provided that they have similar size range, adequate crushstrength to endure the manufacturing process of shingle making, andhaving suitable durability for roofing environments.

In one set of presently preferred embodiments, the inert base particlescomprise solar-reflective particles. Preferably, in this embodiment, theinert base particles are selected from the group consisting of slate,feldspathic rock, plagioclase rock, chert rock, aluminum oxide, mullite,ceramic grog, crushed porcelain, white-pigmented glass, copper, andzinc.

In one set of presently preferred embodiments, the inert base particlescomprise particles having a generally plate-like geometry. Examples ofgenerally plate-like particles include mica and flaky slate. Coloredroofing granules having a generally plate-like geometry have been foundto provide greater surface coverage when used to prepare bituminousroofing products, when compared with conventional “cubical” roofinggranules, as shown in Table 1 below. Granule surface coverage ismeasured using image analysis software, namely, Image-Pro Plus fromMedia Cybernetics, Inc., Silver Spring, Md. 20910. The shingle surfacearea is recorded in a black and white image using a CCD camera fitted toa microscope. The image is then separated into an asphalt coatingportion and a granule covering portion using the threshold method ingray scale. The amount of granule coverage is then calculated by theimage analysis software based upon the number of pixels with gray scaleabove the threshold level divided by the total number of pixels in theimage.

TABLE 1 Sample Color Granule Type Surface Coverage % A White cubical86.0 B Wood Blend cubical 86.6 C Natural flaky slate 91.6 D Naturalflaky slate 92.1 E Natural flaky slate 92.9 F Natural flaky slate 91.8

Roofing granules of the present invention include one or more coatinglayers formed from suitable coating compositions. The coatingcompositions typically include a coating binder in which particulatematerial is dispersed to provide a desired specific function, such as toprovide an aesthetically attractive color, or to provide solar heatreflectance.

Suitable binders for the coating compositions employed in preparingroofing granules according to the present invention can include, but notlimited to, metal-silicates, phosphates, aluminates, silica coating,ceramic glazes, and suitable polymeric binders with good outdoordurability. When metal-silicate binders are used, the said roofinggranules can be manufactured by the traditional method for makingrooting granules as those disclosed in U.S. Pat. No. 2,927,045. Thecoating binders employed in the coating compositions of the presentinvention preferably comprise an aluminosilicate material, such askaolin clay and an alkali metal silicate, such as sodium silicate.Alternatively, the binder, and especially binders employed in preparingcoating compositions for outer or exterior coating layers, can comprisean organic material, such as a curable polymeric material.

Coating binders employed in the processes of the present invention toform coating compositions are preferably formed from a mixture of analkali metal silicate, such as aqueous sodium silicate, andheat-reactive aluminosilicate material, such as clay, preferably,kaolin. The proportion of alkali metal silicate to heat-reactivealuminosilicate material is preferably from about 3:1 to about 1:3 partsby weight alkali metal silicate to parts by weight heat-reactivealuminosilicate material, more preferably about 2:1 to about 0.8:1 partsby weight alkali metal silicate to parts by weight heat-reactivealuminosilicate material. Alternatively, the inert base or coreparticles can be first mixed with the heat-reactive aluminosilicate tocoat the base particles, and the alkali metal silicate can besubsequently added with mixing. When two or more coating layers areformed on the inert base or core particles, the inner and outer coatinglayers can be formed from coating compositions formulated using the sameor similar binders.

When the roofing granules are fired at an elevated temperature, such asat least about 200 degrees C., and preferably about 250 to 500 degreesC., the clay reacts with and neutralizes the alkali metal silicate,thereby insolubilizing the binder. The binder resulting from thisclay-silicate process, believed to be a sodium aluminum silicate, isporous, such as disclosed in U.S. Pat. No. 2,379,358 (incorporatedherein by reference). Alternatively, the porosity of the insolubilizedbinder can be decreased by including an oxygen containing boron compoundsuch as borax in the binder mixture, and firing the granules at a lowertemperature, for example, about 250 degree C. to 400 degrees C., such asdisclosed in U.S. Pat. No. 3,255,031 (incorporated herein by reference).

Examples of clays that can be employed in the process of the presentinvention include kaolin, other aluminosilicate clays, Dover clay,bentonite clay, etc.

Coating binders employed in the present invention can include an alkalimetal silicate such as an aqueous sodium silicate solution, for example,an aqueous sodium silicate solution having a total solids content offrom about 38 percent by weight to about 42 percent by weight, andhaving a ratio of sodium oxide to silicon dioxide of from about 1:2 toabout 1:3.25.

In one aspect of the present invention, it is preferred to select thecomposition of the coating binder to maximize the difference inrefractive index between the coating binder material and the particulatematerial dispersed in the coating binder. For example, the presentinvention provides roofing granules with a pigmented ceramic coatinglayer having enhanced light scattering efficiency. A particularly usefulembodiment provided by the present invention comprises reflectiveroofing granules with enhanced solar reflectance, where the granuleshave a ceramic coating comprising a metal-silicate binder and solarheat-reflecting nanoparticles, and the metal-silicate binder includes atleast one element having an atomic weight less than the atomic weight ofsilicon. These lower atomic weight elements serve to decrease themeasured refractive index of the cured metal-silicate binder.Preferably, the lower atomic weight element or elements are provided insufficient amount to lower the measured refractive index of the curedmetal-silicate binder by at least 0.003 units, more preferably by atleast 0.005 units.

The decrease in the refractive index of the cured metal-silicate binderresults in a greater differential in refractive index between the matrixand the particulate material, such as nanoparticles, in the coatinglayer. This greater refractive index difference increases the lightscattering efficiency of the particulate in the coating layer, thusincreasing the reflectivity of the coating layer. The increasedreflectivity can be in any or all of the visible, near infrared andinfrared spectral ranges. Since the refractive index of a material is aphysical property dependent on the frequency of electromagneticradiation, the composition of the specific layer can be selected toenhance the refractive index difference between the coating binder andthe particulate material dispersed in the coating binder for a specificfrequency range. For example, in the case of a coating layer in whichsolar heat-reflecting nanoparticles are dispersed, the composition ofthe coating binder can be selected to increase the refractive indexdifference in the near infrared spectral range. Similarly, in the caseof a coating layer in which colored nano-pigment particles aredispersed, the composition of the coating binder can be selected toincrease the refractive index difference between the coating binder andthe particulate material dispersed in the coating binder in the visiblespectral range. Thus, roofing granules of the present invention havingcolored coating layers can exhibit more vivid colors than roofinggranules having coatings based on conventional silicate binders.Similarly, roofing granules of the present invention having solarheat-reflective coating layers can exhibit greater solar reflectancethan roofing granules having coating layers based on conventionalsilicate binders.

Organic binders can also be employed in the preparing roofing granulesof the present invention. The use of suitable organic binders, whencured, can also provide superior granule surface with enhanced granuleadhesion to the asphalt substrate and with better staining resistance toasphaltic materials. Roofing granules colored by inorganic binders oftenrequire additional surface treatments to impart certain water repellencyfor granule adhesion and staining resistance. U.S. Pat. No. 5,240,760discloses examples of polysiloxane-treated roofing granules that provideenhanced water repellency and staining resistance. With the organicbinders, the additional surface treatments may be eliminated. Also,certain organic binders, particularly those water-based systems, can becured by drying at much lower temperatures as compared to the inorganicbinders such as metal-silicates, which often require curing attemperatures greater than about 500° C. or by using a separate picklingprocess to render the coating durable.

Examples of organic binders that can be employed in the process of thepresent invention include acrylic polymers, alkyd and polyesters, aminoresins, epoxy resins, phenolics, polyamides, polyurethanes, siliconeresins, vinyl resins, polyols, cycloaliphatic epoxides, polysulfides,phenoxy, fluoropolymer resins. Examples of uv-curable (that is, curableby exposure to ultraviolet radiation) organic binders that can beemployed in the process of the present invention include uv-curableacrylates and uv-curable cycloaliphatic epoxides.

An organic material can be employed as a binder for the coatingcomposition used in the process of the present invention. Preferably, ahard, transparent organic material is employed. Especially preferred areuv-resistant polymeric materials, such as poly(meth)acrylate materials,including poly methyl methacrylate, copolymers of methyl methacrylateand alkyl acrylates such as ethyl acrylate and butyl acrylate, andcopolymers of acrylate and methacrylate monomers with other monomers,such as styrene. Preferably, the monomer composition of the copolymer isselected to provide a hard, durable coating. If desired, the monomermixture can include functional monomers to provide desirable properties,such as crosslinkability to the copolymers. The organic material can bedispersed or dissolved in a suitable solvent, such as coatings solventswell known in the coatings arts, and the resulting solution used to coatthe granules using conventional coatings techniques. Alternatively,water-borne emulsified organic materials, such as acrylate emulsionpolymers, can be employed to coat the granules, and the watersubsequently removed to allow the emulsified organic materials of thecoating composition to coalesce.

Roofing granules according to the present invention can include acolored coating layer in which particles of one or more conventionalmetal oxide pigments are dispersed. Examples of coatings pigments thatcan be used include those provided by the Color Division of FerroCorporation, 4150 East 56th St., Cleveland, Ohio 44101, and producedusing high temperature calcinations, including PC-9415 Yellow, PC-9416Yellow, PC-9158 Autumn Gold, PC-9189 Bright Golden Yellow, V-9186Iron-Free Chestnut Brown, V-780 Black, V0797 IR Black, V-9248 Blue,PC-9250 Bright Blue, PC-5686 Turquoise, V-13810 Red, V-12600 CamouflageGreen, V12560 IR Green, V-778 IR Black, and V-799 Black. Furtherexamples of coatings pigments that can be used include white titaniumdioxide pigments provided by Du Pont de Nemours, P.O. Box 8070,Wilmington, Del. 19880.

Examples of white pigments that can be employed in preparing coatinglayers of the roofing granules of the present invention include rutiletitanium dioxide, anatase titanium dioxide, lithopone, zinc sulfide,zinc oxide, lead oxide, and void pigments such as sphericalstyrene/acrylic beads (Ropaque® beads, Rohm and Haas Company), andhollow glass beads having pigmentary size for increased lightscattering.

Roofing granules according to the present invention can include one ormore coating layers in which are dispersed near infrared-reflectivepigments. Examples of colored infrared-reflective pigments that can beused include infrared-reflective pigments that comprise a solid solutionincluding iron oxide, such as disclosed in U.S. Pat. No. 6,174,360,incorporated herein by reference. Colored infrared-reflective pigmentsthe can be used in the preparing the roofing granules of the presentinvention also include near infrared-reflecting composite pigments suchas disclosed in U.S. Pat. No. 6,521,038, incorporated herein byreference. Composite pigments are composed of a near-infrarednon-absorbing colorant of a chromatic or black color and a white pigmentcoated with the near infrared-absorbing colorant. Near-infrarednon-absorbing colorants that can be used in the present invention areorganic pigments such as organic pigments including azo, anthraquinone,phthalocyanine, perinone/perylene, indigo/thioindigo, dioxazine,quinacridone, isoindolinone, isoindoline, diketopyrrolopyrrole,azomethine, and azomethine-azo functional groups. Preferred blackorganic pigments include organic pigments having azo, azomethine, andperylene functional groups.

Other examples of near infrared-reflective pigments include thoseavailable from the Shepherd Color Company, Cincinnati, Ohio, includingArctic Black 10C909 (chromium green-black), Black 411 (chromium ironoxide), Brown 12 (zinc iron chromite), Brown 8 (iron titanium brownspinel), and Yellow 193 (chrome antimony titanium).

Near infrared-reflective coating compositions according to the presentinvention can also include supplementary pigments to spaceinfrared-reflecting pigments, to reduce absorption bymultiple-reflection. Examples of such “spacing” pigments includeamorphous silicic acid having a high surface area and produced by flamehydrolysis or precipitation, such as Aerosil TT600 supplied by Degussa,as disclosed in U.S. Pat. No. 5,962,143, incorporated herein byreference.

Roofing granules according to the present invention can include one ormore coating layers in which nano pigment particles are dispersed toprovide color while reducing absorption in the visible spectral range incomparison with pigment particles of the same chemical composition buthaving a greater average particle size. Colored nano-pigments that canbe employed in the coating layers of the roofing granules of the presentinvention include colored nano-pigments having an average particle sizeof less than about 200 nanometers and an average absorbency of less thanabout 20 percent in the wavelength range of from 700 to 2500 nanometers.Examples of colored nano-pigments that can be employed in the roofinggranules of the present invention include carbon black, bone black,copper chromite black, iron oxide black, and KFe₂(CN)₆.H₂O.

Roofing granules according to the present invention can also include oneor more coating layers in which solar heat-reflective nanoparticles aredispersed to provide solar heat reflectivity.

Solar heat-reflective roofing granules employing an outer coating layerincluding nanoparticles are preferably provided by coating intermediateparticles with sol-gel coating composition of nanoparticles, preferablytitanium dioxide nanoparticles, and curing the resulting coatedintermediate particles. The sol-gel coating composition preferablycomprises titanium dioxide nanoparticles dispersed in tetraethylorthosilicate, a silicic acid ester coupling agent. Examples of couplingagents that can be employed in preparing the titanium dioxidenanoparticle sol include silicic acid esters such as tetrabutylorthosilicate, tetramethoxysilane, tetra-n-propoxysilane, and oligomerictetraethoxysilane (available under the SIVENTO trademark from DegussaAG, Frankfurt am Main, Germany), alkylalkoxysilanes such asmethyltrimethoxysilane, methyltriethoxysilane, propyltrimethoxysilane,propyltriethoxysilane, isobutyltrimethoxysilane,isobutyltriethoxysilane, octyltrimethoxysilane, octyltriethoxysilane,hexadecyltrimethoxysilane, phenyl trimethoxysilane, andphenyltriethoxysilane, haloalkylalkoxysilanes such astridecafluoro-1,1,2,2-tetrahydrooctyltriethoxysilane, organofunctionalsilanes such as 3-g lycidyloxypropyltrimethoxysilane, 3-glycidyloxypropyltriethoxysilane, 3-methyacryloxypropyltrimethoxysilane,vinyltrimethoxysilane, vinyltriethoxysilane,3-mercaptopropyltrimethoxysilane, and 3-mercaptopropyltriethoxysilane,aminofunctional alkoxysilanes such as 3-aminopropyltriethoxysilane,3-aminopropyltrimethoxysilane,2-aminoethyl-3-aminopropyltrimethoxysilane, triaminofunctionalpropyltrimethoxysilane, N-(n-butyl)-3-aminopropyltrimethoxysilane, and3-aminopropylmethydiethoxysilane, and mixtures thereof. The silanecoupling agent is typically dissolved in an alcohol such as ethanol,isopropanol, methoxypropanol, or an alcoholic mixture, and thenhydrolyzed by addition of water acidified with hydrochloric acid orsulfuric acid, to form a sol-gel coating composition. The titaniumdioxide nanoparticles can be dispersed in the silane/alcohol solutionprior to the addition of the acidified water. The coating compositioncan be applied to the intermediate particles by conventional coatingapplication techniques such as by spraying, dipping, flow coating, andthe coating can be subsequently cured thermally. If desired, suitablecrosslinking agents can be included in the coating composition to effecta room temperature cure, such as by hydrolysis by ambient moisture.

The nanoparticle titanium dioxide can be prepared by hydrolysis fromhydrolyzable titanium compounds, such as for example, titaniumtrichloride, titanium tetrachloride, titanyl sulfate, titanium sulfate,titanium oxysulfate, titanium iron sulfate solution, titaniumoxychloride, as well as titanium alkoxides including titanium ethoxide,titanium ethylhexoxide, titanium isobutoxide, titanium isopropoxide,titanium isopropylate or titanium methoxide. For example, a nanoparticletitanium dioxide sol can be produced by adding titanium isopropoxidedropwise to concentrated hydrochloric acid diluted with a suitable polarorganic solvent such as 2-methoxyethanol. The production of titaniumdioxide nanoparticles is disclosed, for example, in U.S. Pat. Nos.5,840,111, 6,610,135, and 6,653,356.

Titanium dioxide nanoparticles can also be prepared by condensationtechniques, such as a combustion flame—chemical vapor condensationprocess from an organometallic precursor compound, such as disclosed,for example, in U.S. Pat. Nos. 5,514,350 and 5,876,683.

Sol-gel coating systems are well known in the coatings art, and arediscussed, for example, in C. J. Brinker and G. W. Scherer, Sol-GelScience: The Physics and Chemistry of Sol-Gel Processing (Academic PressSan Diego 1989).

In the alternative, the solar heat-reflective roofing granule employingtitanium dioxide nanoparticles are provided by coating intermediateparticle with an outer coating composition comprising titanium dioxidenanoparticles dispersed in an alkali metal silicate binder, and thencuring the alkali metal silicate binder by the application of heat or bychemical means, depending on the binder composition.

Optionally, the coating compositions of the present invention furthercomprise at least one supplementary pigment. Preferably, supplementarypigments having high near infrared transparency are used in preparingcoating layers employed in the roofing granules of the presentinvention. Examples of supplementary pigments include pearlescentpigments, light-interference platelet pigments, ultramarine blue,ultramarine purple, cobalt chromite blue, cobalt aluminum blue, chrometitanate, nickel titanate, cadmium sulfide yellow, cadmium sulfoselenideorange, and organic pigments such as phthalo blue, phthalo green,quinacridone red, diarylide yellow, and dioxazine purple.

Preferred supplementary pigments include pearlescent pigments,light-interference platelet pigments, ultramarine blue, ultramarinepurple, cobalt chromite blue, cobalt aluminum blue, chrome titanate,nickel titanate, cadmium sulfide yellow, cadmium sulfoselenide orange,phthalo blue, phthalo green, quinacridone red, diarylide yellow, anddioxazine purple.

Light-interference platelet pigments are known to give rise to variousoptical effects when incorporated in coatings, including opalescence or“pearlescence.” Surprisingly, light-interference platelet pigments havebeen found to provide or enhance infrared-reflectance of roofinggranules coated with compositions including such pigments.

Examples of light-interference platelet pigments that can be employed inthe process of the present invention include pigments available fromWenzhou Pearlescent Pigments Co., Ltd., No. 9 Small East District,Wenzhou Economical and Technical Development Zone, Peoples Republic ofChina, such as Taizhu TZ5013 (mica, rutile titanium dioxide and ironoxide, golden color), TZ5012 (mica, rutile titanium dioxide and ironoxide, golden color), TZ4013 (mica and iron oxide, wine red color),TZ4012 (mica and iron oxide, red brown color), TZ4011 (mica and ironoxide, bronze color), TZ2015 (mica and rutile titanium dioxide,interference green color), TZ2014 (mica and rutile titanium dioxide,interference blue color), TZ2013 (mica and rutile titanium dioxide,interference violet color), TZ2012 (mica and rutile titanium dioxide,interference red color), TZ2011 (mica and rutile titanium dioxide,interference golden color), TZ1222 (mica and rutile titanium dioxide,silver white color), TZ1004 (mica and anatase titanium dioxide, silverwhite color), TZ4001/600 (mica and iron oxide, bronze appearance),TZ5003/600 (mica, titanium oxide and iron oxide, gold appearance),TZ1001/80 (mica and titanium dioxide, off-white appearance), TZ2001/600(mica, titanium dioxide, tin oxide, off-white/gold appearance),TZ2004/600 (mica, titanium dioxide, tin oxide, off-white/blueappearance), TZ2005/600 (mica, titanium dioxide, tin oxide,off-white/green appearance), and TZ4002/600 (mica and iron oxide, bronzeappearance).

Examples of light-interference platelet pigments that can be employed inthe process of the present invention also include pigments availablefrom Merck KGaA, Darmstadt, Germany, such as Iriodin® pearlescentpigment based on mica covered with a thin layer of titanium dioxideand/or iron oxide; Xirallic™ high chroma crystal effect pigment basedupon Al2O3 platelets coated with metal oxides, including Xirallic T60-10 WNT crystal silver, Xirallic T 60-20 WNT sunbeam gold, andXirallic F 60-50 WNT fireside copper; ColorStream™ multi color effectpigments based on SiO2 platelets coated with metal oxides, includingColorStream F 20-00 WNT autumn mystery and ColorStream F 20-07 WNT violafantasy; and ultra interference pigments based on TiO₂ and mica.

Examples of mirrorized silica pigments that can be employed in theprocess of the present invention include pigments such as Chrom Brite™CB4500, available from Bead Brite, 400 Oser Ave, Suite 600, Hauppauge,N.Y. 11788.

Aluminum oxide, preferably in powdered form, can be used assolar-reflective additive in the color coating formulation to improvethe solar reflectance of colored roofing granules without affecting thecolor. The aluminum oxide should have particle size less than #40 USmesh (425 micrometer), preferably between 0.1 micrometer and 5micrometer. More preferably, the particle size is between 0.3 micrometerand 2 micrometer. The alumina should have percentage Al₂O₃>90%, morepreferably >95%.

In another aspect of the present invention, the colored solarheat-reflective roofing granules are formed from intermediate particles,such as conventional colored roofing granules, comprising inert mineralbase particles coated with a cured first coating composition including afirst or inner coating binder and at least one metal oxide colorant. Theintermediate particles are coated with a cured transparent second orouter coating including at least two thin metal layers. Preferably, thethickness of the second coating formed by the at least two metal layersis selected to maximize infrared reflectivity consistent with achievingthe desired color tone for the roofing granule. Preferably at least onelayer of the second coating is formed from a metal selected from thegroup consisting of silver, gold and copper.

The deposition of thin metal films by a variety of techniques is wellknown in the art. Preferably, each of the layers of thin film is appliedby an application process selected from the group consisting ofatmospheric plasma deposition, plasma-assisted polymerization, chemicalvapor deposition, physical vapor deposition, sputtering, casting,coating, laminating, electroplating, electroless plating, and thermalspraying. Preferably, the application process is selected from the groupconsisting of atmospheric plasma deposition, plasma-assistedpolymerization, and physical vapor deposition.

Preferably, each of the layers of the thin film comprises a materialselected from the group consisting of silver, aluminum, copper, zinc,tin, gold, palladium, nickel, and alloys thereof. Each of the layers ofthin film can comprise an alloy of silver and copper, an alloy of goldand palladium, etc.

In one aspect of the process of the present invention, a clear coatingis applied over the outermost layer of thin film to protect the thinfilm. Preferably, the clear coating is applied by a method selected fromthe group consisting of spraying, electrostatic spraying, sonicspraying, ink jet printing, gravure printing, roll coating, andextrusion coating. Preferably, the clear coating is selected from thegroup consisting of poly(meth)acrylates, polyurethanes, fluoropolymers,phosphates, titanates, zirconates, silicates, and silicas.

Preferably, the outer surface of the intermediate particles is preparedfor application of the first layer of thin film; preferably, bycleaning. Preferably, the intermediate particles are cleaned by aprocess selected from the group consisting of atmospheric pressureplasma cleaning, corona treating, solvent washing, detergent washing,soap washing, high pressure washing, and steam cleaning.

Referring now to the figures in which like reference numerals representlike element in each of the several views, there is shown in FIG. 1, aschematic illustration of the structure of a section of a coloredinfrared-reflective roofing granule 100 according to a presentlypreferred first embodiment of the present invention.

FIG. 1 is a schematic illustration of the structure of a coloredinfrared-reflective roofing granule 100 according to a presentlypreferred first embodiment of the present invention. In this embodiment,the colored solar heat-reflective roofing granules 100 are prepared fromintermediate particles 120 comprising inert mineral base particles 102coated with a cured first coating composition 104 including a first orinner coating binder 106 and at least one metal oxide colorant 108 toform an inner or first coating layer 110.

The intermediate particles 120 are coated with a cured transparentsecond or outer coating composition 130 including a second or outercoating binder 134, and highly reflective nanoparticles 132, such astitanium dioxide nanoparticles to form a second or outer coating layer140. Preferably, the cured second coating composition 130 is transparentto visible radiation, so that the appearance of the coloredinfrared-reflective roofing granules 100 is determined by the metaloxide colorant(s) 108 in the cured first coating composition 104.Preferably, the thickness of the outer coating layer 140 formed by thecured second coating composition 130, the outer coating binder 134, andthe nanoparticles 132 are selected to maximize transparency consistentwith achieving the desired color tone for the roofing granule 100. Theouter coating layer 140 is preferably formed by a sol-gel of titaniumdioxide nanoparticles; however, other types of sufficiently small highlyreflective pigment particles, such as zinc oxide particles, dispersed inother types of coating binders, such as conventional metal silicatebinders, can also be employed. In particular, titanium dioxidenanoparticles dispersed in a conventional metal silicate binder can alsobe used.

Preferably, the hydrophobicity of the outer coating layer 140 isincreased by the addition of a mineral oil or silicone oil coating 142,in order to enhance the adhesion of the roofing granules 100 tobituminous surfaces and to increase the ease of manufacture.

Preferably, in the colored solar heat-reflective roofing granules 100the nanoparticles 132 comprise from about 0.5 percent by weight to about40 percent by weight of the second coating composition 130. Preferably,the nanoparticles 132 have an average particle size or crystal size ofless than about 100 nanometers, and more preferably, of less than about50 nanometers. It is also preferred that the nanoparticles besubstantially free of any material of a size large enough to effectivelyscatter incident light, and thus to contribute opacity to the outercoating layer 140.

Preferably, the second layer comprising the cured second coatingcomposition 140 has an incident radiation transmission coefficient of atleast 60 percent, and more preferably at least 90 percent, in the range400 nanometers to 800 nanometers.

In this first embodiment, the second coating composition 130 comprisesfrom about 2 percent by weight of the intermediate particles 120 toabout 20 percent by weight of the intermediate particles 120, morepreferably, from about 4 percent by weight of the intermediate particles120 to about 10 percent by weight of the intermediate particles 120. Inthis first embodiment, the first or base coating composition 104preferably comprises from about 1 percent by weight of the inert mineralparticles 102 to about 20 percent by weight of the inert mineralparticles 102. In this first embodiment, the inner or first coatingcomposition binder 106 preferably comprises an aluminosilicate materialand an alkali metal silicate, and the aluminosilicate material ispreferably clay, although an organic material can optionally be employedas the first coating composition binder 106.

Thus, in this first embodiment of colored solar heat-reflective roofinggranules 100 according to the present invention, the infrared or solarheat reflectance of the colored roofing granules 100 is attributable totitanium dioxide nanoparticles 132 in the cured outer or second coatingcomposition 130, while the color of the granules 100 is substantiallyattributable to the at least one metal oxide colorant 108 in the curedinner or first coating composition 104.

FIG. 2 is a schematic illustration of the structure of a colored solarheat-reflective roofing granule 160 according to a presently preferredsecond embodiment of the present invention. In this embodiment, thecolored solar heat-reflective roofing granules 160 comprise intermediateparticles 180 which include inert mineral base particles 162 coated witha cured first coating composition 164 including a first or inner coatingbinder 166 and at least one metal oxide colorant 168, to form a first orinner coater layer 170 and the intermediate particles 180 are coatedwith a cured transparent second or outer coating 190 including at leasttwo metal layers, such as the three metal layers 192, 194, 196,exemplified in FIG. 2. Preferably, the thickness of the second coating190 formed by the at least two metal layers 192, 194, 196 is selected tomaximize infrared reflectivity while simultaneously achieving thedesired color tone for the roofing granule 160. Preferably at least onelayer of the second coating 190 is formed from a metal selected from thegroup consisting of silver, gold and copper.

Preferably, the hydrophobicity of the second coating layer 190 isincreased by the addition of a mineral oil or silicone oil 198 coating,in order to enhance the adhesion of the roofing granules 160 tobituminous surfaces and to increase the ease of manufacture.

Preferably, in the colored solar heat-reflective roofing granules 160the thickness of the second coating layer 190 is less than about 50nanometers.

Preferably, the second coating layer 190 has an incident radiationtransmission coefficient of at least 60 percent, and more preferably atleast 90 percent, in the range 400 nanometers to 800 nanometers.

In this second embodiment, the second coating layer 190 comprises fromabout 2 percent by weight of the intermediate particles 180 to about 20percent by weight of the intermediate particles 180, more preferably,from about 4 percent by weight of the intermediate particles 180 toabout 10 percent by weight of the intermediate particles 180. In thissecond embodiment, the first or base coating composition 164 preferablycomprises from about 1 percent by weight of the inert mineral particles162 to about 20 percent by weight of the inert mineral particles 162. Inthis second embodiment, the inner or first coating composition binder166 preferably comprises an aluminosilicate material and an alkali metalsilicate, and the aluminosilicate material is preferably clay, althoughan organic material can optionally be employed as the first coatingcomposition binder 166.

Thus, in this second embodiment of colored solar heat-reflective roofinggranules 160 according to the present invention, the infrared or solarheat reflectance of the colored roofing granules 160 is attributable tothe reflectivity of the thin metal films 192, 194, 196, while the colorof the granules 160 is substantially attributable to the at least onemetal oxide colorant 168 in the cured inner or first coating composition164.

FIG. 3 is a schematic illustration of the structure of a coloredinfrared-reflective roofing granule 200 according to a presentlypreferred third embodiment of the present invention. In this embodiment,the colored solar heat-reflective roofing granules 200 are prepared fromintermediate particles 220 comprising inert mineral base particles 202coated with a cured first coating composition 204 including a first orinner coating binder 206 and highly reflective nanoparticles 208, suchas titanium dioxide nanoparticles, to form a first or inner coatinglayer 210. The inner coating layer 210 is preferably formed by a sol-gelof titanium dioxide nanoparticles; however, other types of sufficientlysmall highly reflective pigment particles, such as zinc oxide particles,dispersed in other types of coating binders, such as conventional metalsilicate binders, can also be employed. In particular, titanium dioxidenanoparticles dispersed in a conventional metal silicate binder can alsobe used. Preferably, the highly reflective pigment particles have anaverage reflectance greater than about 60 percent in the wavelengthrange of from about 700 to 2500 nanometers, and more preferably greaterthan about 80 percent.

The intermediate particles 220 are coated with a cured second or outercoating composition 230 including a second or outer coating binder 234and colored nano-pigment particles 232, such as iron oxidenanoparticles, to form an outer or second coating layer 240. The curedsecond coating composition 230 can be substantially transparent toradiation in the wavelength range from about 700 to 2500 nanometers, sothat solar heat radiation incident upon the outer coating layer 240 istransmitted through the outer coating layer 240 to the inner coatinglayer 210 and reflected by the highly reflective nanoparticles 208 inthe inner coating layer 210 back through the outer coating layer 240.The appearance of the colored infrared-reflective roofing granules 200is determined by the nano-pigment colorant(s) 232 in the cured secondcoating composition 230 forming the outer coating layer 240. Preferably,the thickness of the outer coating layer 240 formed by the cured secondcoating composition 230, the outer coating binder 234, and thenano-pigment particles 232 are selected to achieve the desired colortone for the roofing granule 200.

Preferably, the second or outer layer 240 comprising the cured secondcoating composition 230 has an incident radiation transmissioncoefficient of at least 60 percent, and more preferably at least 90percent, in the range 800 nanometers to 2500 nanometers.

In this third embodiment, the second coating composition 230 comprisesfrom about 2 percent by weight of the intermediate particles 220 toabout 20 percent by weight of the intermediate particles 220, morepreferably, from about 4 percent by weight of the intermediate particles220 to about 10 percent by weight of the intermediate particles 220. Inthis third embodiment, the first or base coating composition 204preferably comprises from about 1 percent by weight of the inert mineralparticles 202 to about 20 percent by weight of the inert mineralparticles 202. In this third embodiment, the inner or first coatingcomposition binder 206 preferably comprises an aluminosilicate materialand an alkali metal silicate, and the aluminosilicate material ispreferably clay, although an organic material can optionally be employedas the first coating composition binder 206.

Thus, in this third embodiment of colored solar heat-reflective roofinggranules 200 according to the present invention, the infrared or solarheat reflectance of the colored roofing granules 200 is attributable tothe titanium dioxide nanoparticles 208 in the cured inner or firstcoating composition 204 forming the inner coating layer 210, while thecolor of the granules 200 is substantially attributable to the at leastone nano-pigment colorant 232 in the cured outer or second coatingcomposition 230 forming the outer coating layer 240.

FIG. 4 is a schematic illustration of the structure of a coloredinfrared-reflective roofing granule 260 according to a presentlypreferred fourth embodiment of the present invention. In thisembodiment, the colored solar heat-reflective roofing granules 260 areprepared from inert mineral base particles 262 formed from a solarheat-reflective material, such as slate, feldspathic rock, plagioclaserock, chert rock, aluminum oxide, mullite, ceramic grog, crushedporcelain, white-pigmented glass, copper, and zinc. Preferably, thesolar-reflective inert base particles have a solar reflectivity of atleast 60 percent. Preferably, the solar heat-reflective base particleshave an average reflectance greater than about 60 percent in thewavelength range of from about 700 to 2500 nanometers, and morepreferably greater than about 80 percent.

The solar heat-reflective base particles 262 are coated with a curedouter coating composition 270 including an outer coating binder 274 andcolored nano-pigment particles 272, such as iron oxide nanoparticles, toform an outer coating layer 280. The cured coating composition 270 canbe substantially transparent to radiation in the wavelength range fromabout 700 to 2500 nanometers, so that solar heat radiation incident uponthe outer coating layer 280 is transmitted through the outer coatinglayer 280 to the surface 264 of the solar heat-reflective base particles262 and reflected by the solar heat-reflective base particles 262 backthrough the outer coating layer 280. The appearance of the coloredinfrared-reflective roofing granules 260 is determined by thenano-pigment colorant(s) 272 in the cured outer coating composition 270forming the outer coating layer 280. Preferably, the thickness of theouter coating layer 280 formed by the cured outer coating composition270, the outer coating binder 274, and the nano-pigment particles 272 isselected to achieve the desired color tone for the roofing granule 260.

Thus, in this fourth embodiment of colored solar heat-reflective roofinggranules 260 according to the present invention, the infrared or solarheat reflectance of the colored roofing granules 260 is substantiallyattributable to the solar heat-reflective base particles 262, while thecolor of the granules 260 is substantially attributable to the at leastone nano-pigment colorant 272 in the cured outer or second coatingcomposition 270 forming the outer coating layer 280.

FIG. 5 is a schematic illustration of the structure of a coloredinfrared-reflective roofing granule 300 according to a presentlypreferred fifth embodiment of the present invention. In this embodiment,the colored solar heat-reflective roofing granules 300 are prepared frominert mineral base particles 302 formed from a non-solar heat-reflectivematerial.

The inert base particles 302 are coated with a cured coating composition310 including a coating binder 304, and pigments 314 including colorednano-pigment particles 312, such as iron oxide nanoparticles, and solarheat-reflective nanoparticles 318, such as nanoparticle titaniumdioxide, to form an outer coating layer 320. The cured coatingcomposition 310 can be substantially reflective to radiation in thewavelength range from about 700 to 2500 nanometers, so that solar heatradiation incident upon the outer coating layer 320 is reflected by thesolar heat-reflective nanoparticles 318. The appearance of the coloredinfrared-reflective roofing granules 300 is determined by thenano-pigment colorant(s) 312 in the cured coating composition 310forming the outer coating layer 320. Preferably, the thickness of theouter coating layer 320 formed by the cured coating composition 310, theouter coating binder 314, and the nano-pigment particles 318 is selectedto achieve the desired color tone for the roofing granule 300.

Thus, in this fifth embodiment of colored solar heat-reflective roofinggranules 300 according to the present invention, the infrared or solarheat reflectance of the colored roofing granules 300 is substantiallyattributable to the solar heat-reflective nanoparticles 318, while thecolor of the granules 300 is substantially attributable to the at leastone nano-pigment colorant 312 in the cured outer coating composition 310forming the outer coating layer 320.

FIG. 6 is a schematic illustration of the structure of a coloredinfrared-reflective roofing granule 340 according to a presentlypreferred sixth embodiment of the present invention. In this embodiment,the colored solar heat-reflective roofing granules 340 are prepared fromintermediate particles 360 comprising inert mineral base particles 342coated with a cured first coating composition 344 including a first orinner coating binder 346 and highly reflective nanoparticles 348, suchas titanium dioxide nanoparticles, to form a first or inner coatinglayer 350. The inner coating layer 350 is preferably formed by a sol-gelof titanium dioxide nanoparticles; however, other types of sufficientlysmall highly reflective pigment particles, such as zinc oxide particles,dispersed in other types of coating binders, such as conventional metalsilicate binders, can also be employed. In particular, titanium dioxidenanoparticles dispersed in a conventional metal silicate binder can alsobe used. Preferably, the highly reflective pigment particles have anaverage reflectance greater than about 60 percent in the wavelengthrange of from about 700 to 2500 nanometers, and more preferably greaterthan about 80 percent.

The intermediate particles 360 are coated with a cured second or outercoating composition 370 including a second or outer coating binder 374and colored nano-pigment particles 372, such as iron oxidenanoparticles, and at least one supplementary pigment 376 to form anouter or second coating layer 380. The at least one supplementarypigment can be selected from the group consisting of pearlescentpigments, light-interference platelet pigments, ultramarine blue,ultramarine purple, cobalt chromite blue, cobalt aluminum blue, chrometitanate, nickel titanate, cadmium sulfide yellow, cadmium sulfoselenideorange, phthalo blue, phthalo green, quinacridone red, diarylide yellow,and dioxazine purple. The cured second coating composition 370 can besubstantially transparent to radiation in the wavelength range fromabout 700 to 2500 nanometers, so that solar heat radiation incident uponthe outer coating layer 380 is transmitted through the outer coatinglayer 380 to the inner coating layer 350 and reflected by the highlyreflective nanoparticles 348 in the inner coating layer 350 back throughthe outer coating layer 380. The appearance of the coloredinfrared-reflective roofing granules 340 is determined by thenano-pigment colorant(s) 372 and the supplementary pigment(s) 376 in thecured second coating composition 370 forming the outer coating layer380. Preferably, the thickness of the outer coating layer 380 formed bythe cured second coating composition 370, the outer coating binder 374,the nano-pigment particles 372, and the supplementary pigment particles378, is selected to achieve the desired color tone for the roofinggranule 360.

Preferably, the second or outer layer 380 comprising the cured secondcoating composition 370 has an incident radiation transmissioncoefficient of at least 60 percent, and more preferably at least 90percent, in the range 800 nanometers to 2500 nanometers.

In this sixth embodiment, the second coating composition 370 comprisesfrom about 2 percent by weight of the intermediate particles 360 toabout 20 percent by weight of the intermediate particles 360, morepreferably, from about 4 percent by weight of the intermediate particles360 to about 10 percent by weight of the intermediate particles 360. Inthis sixth embodiment, the first or base coating composition 344preferably comprises from about 1 percent by weight of the inert mineralparticles 342 to about 20 percent by weight of the inert mineralparticles 342. In this sixth embodiment, the inner or first coatingcomposition binder 346 preferably comprises an aluminosilicate materialand an alkali metal silicate, and the aluminosilicate material ispreferably clay, although an organic material can optionally be employedas the first coating composition binder 346.

Thus, in this sixth embodiment of colored solar heat-reflective roofinggranules 340 according to the present invention, the infrared or solarheat reflectance of the colored roofing granules 340 is attributable tothe titanium dioxide nanoparticles 348 in the cured inner or firstcoating composition 344 forming the inner coating layer 350, while thecolor of the granules 340 is substantially attributable to the at leastone nano-pigment colorant 372 and the at least one supplementary pigment376 in the cured outer or second coating composition 374 forming theouter coating layer 380.

Thus, in one aspect of the present invention, roofing granules with highsolar reflectance are prepared dispersing nano-sized color pigments in abinder to form an outer coating composition. The outer coatingcomposition is applied over a reflective core particle, or optionallyover a core particle that has been coated using a solar reflective baseor inner coating composition. Nano-sized color pigments are known tohave limited transparency in the color visible spectrum from 360 nm-700nm, and this transparency can be employed to provide desirable coloreffects, as in the case of staining wood substrates to reveal the woodgrains. However, nano-sized color pigments also exhibit transparency inthe near infrared range (“NIR”) of solar spectrum ranging from 700nm-2500 nm. Thus, in roofing granules prepared in accordance with thisaspect of the present invention, a portion of the solar radiation in theNIR range is reflected by the reflective substrate formed by the innercoating or reflective base particle, without the adverse effect on colorprovided by the nano-sized color pigments in the outer coating.Furthermore, when selected nano-sized color pigments are dispersed in ametal-silicate binder and applied over a white, titanium dioxidepigmented base coat, enhanced colors or metallic effects are provided.In addition, nano-sized color pigments can be selected to provideadditional surface functionalities, such as algaecidal and/orphoto-catalytic effects. In preparing outer coating compositionsaccording to this aspect of the present invention, in addition tonano-sized color-pigment, the outer coating composition can includeother colorants to produce desirable-colors. In particular, outercoating compositions can include both nano-sized color pigments andpigments of high NIR transparency and/or pigments of high IRreflectivity to produce colored roofing granules with high solarreflectance. Preferably, the binder employed in the other coatingcomposition including nano-sized color pigment is a metal-silicatebinder that has reduced refractive index to further enhance their colorand solar reflectance. The nano-sized color pigments preferably haveparticle sizes in the range from about 20 nm and 150 nm and should haveadequate light-fastness for exterior applications.

Examples of nano-sized color pigments include, but are not limited to,iron oxides, titanates, chrome oxides, zinc ferrites, mixed metaloxides, titanium dioxides, zinc oxides, copper oxides, vanadiumdioxides, magnesium oxides and the halogen adducts, etc. Such nano-sizedcolorants can be dispersed in a binder system through various means toform a durable color coating suitable for roofing granule applications.Many so-called “hot pigments” in the database established by LBL Lab,that is, those pigments with significant absorption in the solarradiation, can become effective “cool pigments” when their sizes arereduced into the nano-sized pigment range.

To prepare solar-reflective roofing granules according to this aspect ofthe present invention, inert mineral core particles can be coated usinga first or inner coating composition having binder formed from ametal-silicate and kaolin clay in which is dispersed a highlysolar-radiation-reflective white pigment, such as rutile titaniumdioxide to form a first or inner coating layer on the mineral coreparticles. The first or inner coating composition is then cured byheating the coated mineral core particles at an elevated temperature torender the binder insoluble to form a cured white-pigmented, solarreflective inner coating layer on the mineral core particles. It ispreferred that the white-pigmented inner coating result provide aparticulate with solar reflectance greater than 40% as measured by theASTM C1549 method. Secondly, the granules with white-pigmented innercoating are then preferably coated with a second or outer coating ofnano-sized colorants dispersed in a metal-silicate binder without thepresence of clay. The second or outer coating composition can alsoinclude other color pigments, IR reflective pigments, IR reflectivefillers, and/or other functional additives. The roofing granules withthe second coating are then again heated at an elevated temperature tocure the second coating composition. In addition, latent reactants mayalso be included in the second coating composition. In addition, or inthe alternative, or the process of acid wash (pickling) may be used tofurther improve the durability of the said granules. The resultingroofing granules can then be surface-treated such as disclosed in U.S.Pat. No. 5,484,477 to provide desirable surface functionalities, andsubsequently can be used in a conventional process for making asphaltshingles.

Advantageously, intermediate particles produced according to one of theembodiments of the present invention described above can be coated withdifferent outer coating compositions if desired. For example,intermediate particles prepared according to the third embodiment have afirst or inner coating comprising highly reflective nanoparticles, and asecond or outer coating composition comprising nano-pigment colorants.Thus, a batch of intermediate particles can be divided into two or moresub-batches, and each sub-batch can be coated with outer coatingcompositions comprising different nano-pigment colorants, such as bluenano-pigment colorants, green nano-pigment colorants, red nano-pigmentcolorants, and the like, to provide roofing granules having a variety ofdifferent colors. Similarly, intermediate particles prepared accordingto the sixth embodiment can be divided into sub-batches, each of whichcan be coated with an outer coating composition comprising a differentcolored nano-pigment, a different supplementary pigment, or a differentcombination of colored nano-pigment and supplementary pigment, toprovide colored roofing granules of differing appearance.

The present invention also provides a process for increasing theinfrared or solar heat reflectance of conventional colored roofinggranules. Conventional colored roofing granules are coated with acoating composition including a coating binder and at least one solarheat-reflective nanoparticle, such as solar heat-reflective titaniumdioxide nanoparticles having an average crystal size less than about 100nanometers, and preferably having an average crystal size less thanabout 50 nanometers. Preferably, the near-infrared reflectance of theconventional colored roofing granules is increased by at least about 20percent, more preferably at least about 25 percent, while substantiallymaintaining the color of the roofing granules, such that the value ofthe total color difference ΔE* is no more than 10 units, more preferablyno more than 5 units, and even more preferably no more than 3 units.

The process of the present invention for producing nearinfrared-reflective roofing granules comprises several steps. In onestep of the present process, suitable base particles are provided. Thesecan be suitably sized, chemically inert, mineral particles. In someembodiments of the present invention, these base or core particles areselected from materials having a high near infrared reflectance. In oneaspect of the present invention, the base particles are coated with aninitial coating composition containing at least one conventional roofinggranule pigment such as a metal oxide and/or at least one colorednano-pigment to form intermediate particles with an inner or firstcoating layer on the base particles. These intermediate particles arethen provided with a second or outer coating layer providing nearinfrared-reflectance, but which is substantially transparent in thevisible region of the electromagnetic spectrum, such as a coating layerin which are dispersed nanoparticles of titanium dioxide, or one or morelayers of a suitable metal film. In another aspect of the presentinvention, the base or core particles are coated with an initial coatingcomposition including a highly near-infrared reflective pigment, such astitanium dioxide nano-particles to form intermediate particles. Theintermediate particles are then coated using a second coatingcomposition including a binder, and at least one color nano-pigment toprovide an outer coating layer that is substantially transparent in thenear infrared portion of the spectrum, while absorbing in the visibleportion of the spectrum to provide the desired color to the roofinggranules. In yet another aspect of the present invention, both colorednano-pigment particles and near infrared-reflecting nanoparticles suchas titanium dioxide nanoparticles are dispersed in a single coatingcomposition, and the coating composition is applied to suitable base orcore mineral particles and cured to provide a coating layer includingboth colored nano-pigment particles and near-infrared reflectingnanoparticles such as titanium dioxide nanoparticles.

Preferably, the at least one infrared-reflective pigment comprises fromabout 1 percent by weight to about 60 percent by weight of the coatingcomposition. It is preferred that the coating composition comprises fromabout 2 percent by weight of the base particles to about 20 percent byweight of the base particles. More preferably, the coating compositioncomprises from about 4 percent by weight of the base particles to about10 percent by weight of the base particles. The coating composition iscured to provide a layer of near infrared-reflective coating material.

Preferably, the near infrared-reflective coating is provided in athickness effective to render the coating opaque to infrared radiation,such as a coating thickness of at least about 100 micrometers. However,advantageous properties of the present invention can be realized withsignificantly lower coating thicknesses, such as at a coating thicknessof from about 2 micrometers to about 25 micrometers, including at acoating thickness of about 5 micrometers.

Preferably, the at least one colored nano-pigment comprises from about0.5 percent by weight to about 40 percent by weight of the coatingcomposition in which the at least one colored nano-pigment is dispersed.It is also preferred that this coating composition comprises from about2 percent by weight of the inert mineral particles to about 20 percentby weight of the inert mineral particles. Preferably, this coatingcomposition forms a layer having sufficient thickness to provide goodhiding and opacity in the visible range of the electromagnetic spectrum,such as a thickness of from about 5 micrometers to about 50 micrometers.

The solar heat reflectance properties of the solar heat-reflectiveroofing granules of the present invention are determined by a number offactors, including the type and concentration of the solarheat-reflective pigment(s) used in the solar heat-reflective coatingcomposition, whether a base coating is employed, and if so, the type andconcentration of the reflective pigment employed in the base coating,the nature of the binder(s) used in for the solar heat-reflectivecoating and the base coating, the number of coats of solarheat-reflective coating employed, the thickness of the solarheat-reflective coating layer and the base coating layer, and the sizeand shape of the base particles.

The present invention provides mineral surfaced asphalt shingles with L*less than 85, and more preferably less than 55, and solar reflectancegreater than 25%. Preferably, asphalt shingles according to the presentinvention comprise colored, infrared-reflective granules according tothe present invention, and optionally, conventional colored roofinggranules. Conventional colored roofing granules and infrared-reflectiveroofing granules can be blended in combinations to generate desirablecolors. The blend of granules is then directly applied on to hot asphaltcoating to form the shingle. Examples of granule deposition apparatusthat can be employed to manufacture asphalt shingles according to thepresent invention are provided, for example, in U.S. Pat. Nos.4,583,486, 5,795,389, and 6,610,147, and U.S. Patent ApplicationPublication U.S. 2002/0092596.

The colored, solar heat-reflective roofing granules prepared accordingto the present invention can be employed in the manufacture of solarheat-reflective roofing products, such as solar heat-reflective asphaltshingles, using conventional roofing production processes. Typically,bituminous roofing products are sheet goods that include a non-wovenbase or scrim formed of a fibrous material, such as a glass fiber scrim.The base is coated with one or more layers of a bituminous material suchas asphalt to provide water and weather resistance to the roofingproduct. One side of the roofing product is typically coated withmineral granules to provide durability, reflect heat and solarradiation, and to protect the bituminous binder from environmentaldegradation. The colored, solar heat-reflective granules of the presentinvention can be mixed with conventional roofing granules, and thegranule mixture can be embedded in the surface of such bituminousroofing products using conventional methods. Alternatively, the colored,solar heat-reflective granules of the present invention can besubstituted for conventional roofing granules in manufacture ofbituminous roofing products to provide those roofing products with solarreflectance.

Bituminous roofing products are typically manufactured in continuousprocesses in which a continuous substrate sheet of a fibrous materialsuch as a continuous felt sheet or glass fiber mat is immersed in a bathof hot, fluid bituminous coating material so that the bituminousmaterial saturates the substrate sheet and coats at least one side ofthe substrate. The reverse side of the substrate sheet can be coatedwith an anti-stick material such as a suitable mineral powder or a finesand. Roofing granules are then distributed over selected portions ofthe top of the sheet, and the bituminous material serves as an adhesiveto bind the roofing granules to the sheet when the bituminous materialhas cooled. The sheet can then be cut into conventional shingle sizesand shapes (such as one foot by three feet rectangles), slots can be cutin the shingles to provide a plurality of “tabs” for ease ofinstallation, additional bituminous adhesive can be applied in strategiclocations on the top or bottom of the shingles and covered with releasepaper, strips or tape to provide for securing successive courses ofshingles during roof installation, and the finished shingles can bepackaged. More complex methods of shingle construction can also beemployed, such as building up multiple layers of sheet in selectedportions of the shingle to provide an enhanced visual appearance, or tosimulate other types of roofing products. Alternatively, the sheet canbe formed into membranes or roll goods for commercial or industrialroofing applications.

The bituminous material used in manufacturing roofing products accordingto the present invention is derived from a petroleum-processingby-product such as pitch, “straight-run” bitumen, or “blown” bitumen.The bituminous material can be modified with extender materials such asoils, petroleum extracts, and/or petroleum residues. The bituminousmaterial can include various modifying ingredients such as polymericmaterials, such as SBS (styrene-butadiene-styrene) block copolymers,resins, flame-retardant materials, oils, stabilizing materials,anti-static compounds, and the like. Preferably, the total amount byweight of such modifying ingredients is not more than about 15 percentof the total weight of the bituminous material. The bituminous materialcan also include amorphous polyolefins, up to about 25 percent byweight. Examples of suitable amorphous polyolefins include atacticpolypropylene, ethylene-propylene rubber, etc. Preferably, the amorphouspolyolefins employed have a softening point of from about 130 degrees C.to about 160 degrees C. The bituminous composition can also include asuitable filler, such as calcium carbonate, talc, carbon black, stonedust, or fly ash, preferably in an amount from about 10 percent to 70percent by weight of the bituminous composite material.

The following examples are provided to better disclose and teachprocesses and compositions of the present invention. They are forillustrative purposes only, and it must be acknowledged that minorvariations and changes can be made without materially affecting thespirit and scope of the invention as recited in the claims that follow.

Example 1

1000 g of #93 roofing granules without any surface treatment (availablefrom CertainTeed Corp. Norwood, Mass.) is first blended with 36.25 g ofsodium silicate (grade 42, Oxychem Corp., Dallas, Tex.), 12.5 g oftitanium dioxide (R101 from DuPont Corp. Wilmington, Del.), 12.5 g ofkaolin clay, and 8.2 g of water in a tumbler to form a uniform coatingon the roofing granules. The coated granules were then dried in afluidized bed and were heated to 925 degrees F. in a rotary kiln toinsolubilize the coating. After cooling to room temperature, thegranules have a white appearance with L*=75.16, a*=−0.33, b*=1.93 asmeasured by HunterLab XE spectrophotometer, and a solar reflectance of44% as measured by the D&S portable reflectometer according to ASTMC1549. The granules were then coated with a second coating compositionconsisting of nano-sized iron oxide pigments of 0.025 g T-3070B, 0.1 gT-2050R, and 0 46 g T-1030Y from Novant Chemicals, 31.25 g of sodiumsilicate, 2.8 g of aluminum fluoride, 0 812 g of sodium silicofluoride,and 7.0 g of water. The second coating composition was cured at atemperature of 450˜475 degrees F. The final granules have a colorreading of L*=61.34, a*=13.06 b*=23.17, and a high solar reflectance of35%, as compared to roofing granules made from traditional processhaving a solar reflectance of 20-25% in the similar color range. Thefinished granules also have a very desirable metallic effect.

Examples 2-4b

The transparency of the nano-sized color pigments in the spectrum rangeof the solar radiation employed in the coating compositions of thepresent invention is shown using the drawdown method typically used inthe coating industry. Results are displayed in Table 2, in which theeffect of employing nano-sized iron oxide pigments in the roofinggranule coating composition is compared with the use of traditional ironoxide pigments using coating drawdown method. In these examples, 2 g ofpigment was mixed with 20 g of sodium silicate under an electric stirrerat 300 rpm until a uniform mixture was formed. The resultant coatingslurry was then formed into a film using a 6 mil drawdown bar (SAR-5T30from BYK Gardner, Columbia, Md.) over an opacity chart paper (SAR-3721from BYK Gardner). The results in Table 1 clearly show the transparencyof nano-sized pigments and their high solar reflectance over whitereflective background, with enhanced color values in combination ofwhite background. However, the same type of iron oxide pigments withlarger particle sizes as in the traditional pigments have good hidingpowder but result in low solar reflectance.

TABLE 2 Example or Comparative Solar Example Pigment Substrate L* a* b*reflectance Comp. Control—standard Black 12.05 11.70 12.90 9.1% Ex. 1airon oxide brown (I-4650 from Rockwood) Comp. Control—standard White11.98 11.61 112.06 13.4% Ex 1b iron oxide brown (I-4650 from Rockwood)Comp. Control—standard Black 32.17 39.06 33.13 27.7% Ex. 2a iron oxidered pigment (120N from Bayer Corp.) Comp. Control—standard White 32.0639.12 33.36 37.6% Ex. 2b iron oxide red pigment (120N from Bayer Corp.)Example Nano pigment—iron Black 17.38 18.41 9.58 11.6% 2a oxide red; T-2050R from Novant Chemicals Example Nano pigment—iron White 20.01 22.2411.26 32.3% 2b oxide red; T- 2050R from Novant Chemicals Example Nanopigment—iron Black 22.02 15.87 16.24 9.4% 3a oxide brown; T- 3070B fromNovant Chemicals Example Nano pigment—iron White 33.44 29.44 30.70 38.1%3b oxide brown; T- 3070B from Novant Chemicals Example Nano pigment—ironBlack 25.58 10.12 20.3 8.7% 4a oxide yellow; T-1030Y from NovantChemicals Example Nano pigment—iron White 47.29 30.09 54.598 40.8% 4boxide yellow; T-1030Y from Novant Chemicals

Various modifications can be made in the details of the variousembodiments of the processes, compositions and articles of the presentinvention, all within the scope and spirit of the invention and definedby the appended claims.

We claim:
 1. Solar heat-reflective roofing granules comprising: (a) abase particle comprising an inert mineral; (b) a first coating on thebase particle, and (c) a second coating on the first coating, whereinthe first coating comprises a first coating binder and particles of atleast one solar reflecting pigment having an average reflectance ofgreater than about 60 percent in the wavelength range of from about 700to 2500 nanometers, and the second coating comprises a second coatingbinder and at least one nano-pigment having an average particle size ofless than about 200 nanometers and an average absorbency of less thanabout 20 percent in the wavelength range of from 700 to 2500 nanometers.2. Solar heat-reflective roofing granules according to claim 1 whereinthe solar reflecting pigment particles have an average solarreflectivity of at least 80 percent in the wavelength range from 700 to2500 nanometers.
 3. Solar heat-reflective roofing granules according toclaim 1 wherein the solar reflecting pigment particles are selected fromthe group consisting of titanium dioxide, zinc dioxide, and zincsulfide.
 4. Solar heat-reflective roofing granules according to claim 1wherein the second coating binder comprises a metal silicate binderhaving a refractive index of less than about 1.50, the silicate coatingbinder including at least one low atomic weight element, other thanoxygen or hydrogen, having an average atomic weight less than theaverage atomic weight of silicon, the at least one low atomic weightelement being present in sufficient amount in the coating binder toreduce the refractive index of the binder by at least about 0.003 units.5. Solar heat-reflective roofing granules according to claim 1, thesecond coating further comprising at least one supplementary pigmenthaving a particle size of greater than about 200 nanometers an averageabsorbency of less than about 20 percent in the wavelength range of from700 to 2500 nanometers.
 6. Solar heat-reflective roofing granulesaccording to claim 5 wherein the at least one supplementary pigment isselected from the group consisting of pearlescent pigments,light-interference platelet pigments, ultramarine blue, ultramarinepurple, cobalt chromite blue, cobalt aluminum blue, chrome titanate,nickel titanate, cadmium sulfide yellow, cadmium sulfoselenide orange,phthalo blue, phthalo green, quinacridone red, diarylide yellow, anddioxazine purple.
 7. Solar heat-reflective roofing granules according toclaim 1 wherein the at least one nano-pigment has an average particlesize of from about 20 to 150 nanometers.
 8. Solar heat-reflectiveroofing granules according to claim 7 wherein the nano-pigment isselected from the group consisting of iron oxides, metal titanates,chromium oxides, zinc ferrites, mixed metal oxides, titanium dioxide,zinc oxides, copper oxides, vanadium oxide, magnesium oxide and thehalogen adducts.
 9. Solar heat-reflective roofing granules according toclaim 1 wherein the nano-pigment is selected from the group of pigmentsthat have strong near infrared absorbency in macro-pigment form. 10.Solar heat-reflective roofing granules according to claim 9 wherein theat least one nano-pigment is selected from the group consisting ofcarbon black, bone black, copper chromite black, iron oxide black, andKFe₂(CN)₆.H₂O.
 11. Solar heat-reflective roofing granules comprising:(a) a solar-reflective inert base particle; and (b) a coating over thebase particle, the coating comprising a binder and at least onenano-pigment having an average particle size of less than about 200nanometers and an average absorbency of less than about 20 percent inthe wavelength range of from 700 to 2500 nanometers.
 12. Solarheat-reflective roofing granules according to claim 11 wherein thesolar-reflective inert base particles have a solar reflectivity of atleast 60 percent.
 13. Solar heat-reflective roofing granules accordingto claim 11 wherein the solar-reflective inert base particles areselected from the group consisting of slate, feldspathic rock,plagioclase rock, chert rock, aluminum oxide, mullite, ceramic grog,crushed porcelain, white-pigmented glass, copper, and zinc.
 14. Solarheat-reflective roofing granules according to claim 11 wherein thesecond coating binder comprises a metal silicate binder having arefractive index of less than about 1.50, the silicate coating binderincluding at least one low atomic weight element, other than oxygen orhydrogen, having an average atomic weight less than the average atomicweight of silicon, the at least one low atomic weight element beingpresent in sufficient amount in the coating binder to reduce therefractive index of the binder by at least about 0.003 units.
 15. Solarheat-reflective roofing granules according to claim 11, the coatingfurther comprising at least one supplementary pigment having a particlesize of greater than about 200 nanometers and an average absorbency ofless than about 20 percent in the wavelength range of from 700 to 2500nanometers.
 16. Solar heat-reflective roofing granules according toclaim 15 wherein the at least one supplementary pigment is selected fromthe group consisting of pearlescent pigments, light-interferenceplatelet pigments, ultramarine blue, ultramarine purple, cobalt chromiteblue, cobalt aluminum blue, chrome titanate, nickel titanate, cadmiumsulfide yellow, cadmium sulfoselenide orange, phthalo blue, phthalogreen, quinacridone red, diarylide yellow, and dioxazine purple. 17.Solar heat-reflective roofing granules according to claim 11 wherein theat least one nano-pigment has an average particle size of from about 20to 150 nanometers.
 18. Solar heat-reflective roofing granules accordingto claim 17 wherein the nano-pigment is selected from the groupconsisting of iron oxides, metal titanates, chromium oxides, zincferrites, mixed metal oxides, titanium dioxide, zinc oxides, copperoxides, vanadium oxide, magnesium oxide and the halogen adducts. 19.Solar heat-reflective roofing granules according to claim 11 wherein thenano-pigment is selected from the group of pigments that have strongnear infrared absorbency in macro-pigment form.
 20. Solarheat-reflective roofing granules according to claim 19 wherein the atleast one nano-pigment is selected from the group consisting of carbonblack, bone black, copper chromite black, iron oxide black, andKFe₂(CN)₆.H₂O.